Hugs Kiwi
All sorts of things can prevent you from hearing the heartbeat in the 1st Trimester and even further along. Anterior placenta as Avid said, also the position of baby and what way they are facing. Even the shape of your uterus
I can understand it being frustrating when your first bubs was easy to find early on though! I would feel the same. I was listening to my son well before 12 weeks, for peace of mind.
If I was you I'd request a dating scan, say it's too stressful not knowing any longer
and stress isn't good for baby!